1. Field of the Invention
The present invention relates to a magnetic head, magnetic head assembly and magnetic recording and reproducing apparatus, and more particularly to a magnetic head, magnetic head assembly and magnetic recording and reproducing apparatus in a microwave assisted recording method.
2. Description of the Related Art
The demand placed on a hard disk unit, which is a magnetic recording and reproducing apparatus, is to improve performance of a magnetic recording medium and a magnetic head in order to increase a magnetic recording density. To increase the recording density of a magnetic recording medium, the size of each magnetic particle in a recording layer needs to be reduced to assure a signal-to-noise ratio (SNR) high enough for reproducing. If the size of the magnetic particle is reduced, however, the volume of the magnetic particle is reduced, causing magnetization to be likely to disappear due to heat fluctuation.
To prevent this and maintain a stable recording state, it is necessary to increase magnetic anisotropy energy (sometimes simply referred to below as Ku) of the magnetic particle in the recording layer. As for a magnetic particle having uniaxial magnetic anisotropy, a magnetic field having intensity necessary for magnetization reversal is referred to as a magnetic anisotropy field Hk, which is represented as the following equation: Hk=2 Ku/Ms, where Ms is saturation magnetization. If a material having large Ku, therefore, Hk becomes large, so a strong recording magnetic field is needed for recording on a magnetic recording medium.
To have a strong recording magnetic field, saturation magnetic flux density Bs of the main magnetic pole of a recording element needs to be increased. As indicated by a Slater-Pauling curve, however, the value of saturation magnetic flux density is peaked at about 2.4 T.
To overcome this situation, energy assisted recording is proposed in which auxiliary energy is given to a medium during recording to lower an effective recording magnetic field strength. A recording method in which a microwave magnetic field is used as an auxiliary energy source is referred to as microwave assisted magnetic recording (MAMR). MAMR is undergoing research and development toward its practical use.
In microwave assisted magnetic recording, when a microwave magnetic field is applied in a medium in-plane direction at the frequency according to an effective magnetic field (Heff) (obtained by adding the magnetic anisotropy field of the medium and a magnetic field generated from the main magnetic pole together) involved in magnetization in the recording layer, precession movement in magnetization in the recording layer is oscillated and the recording capability is assisted.
In an example of an MAMR method, a main magnetic pole forming a recording element of a magnetic head and a spin torque oscillator (STO) structured with multi-layer magnetic thin films in a magnetic gap in a trailing shield are formed, after which a microwave magnetic field is generated in an in-plane direction by self oscillation of the STO and the generated microwave magnetic field is applied to a magnetic recoding medium to induce precession movement in magnetization in the recording layer and assist magnetization reversal in a perpendicular direction (see Japanese Unexamined Patent Application Publication No. 2009-070541, for example).
However, the STO has serious technical problems such as: multi-layer magnetic thin films need to be laminated, making the process complex; to allow the STO to oscillate, a magnetic thin film having extremely high magnetic anisotropy is required; since the oscillation frequency rapidly changes due to a change in the strength of a magnetic field applied to the STO, it is difficult to control the oscillation frequency; and when the density of a current supplied into the STO is increased, the strength of the magnetic field is also increased, but the oscillation frequency is also changed, making it difficult to control these two parameters arbitrarily.
In contrast to a self-oscillating microwave assisted magnetic recording method, a head apparatus is proposed in which a current wire is provided in the vicinity of the main magnetic pole of a magnetic head so that a high-frequency in-plane magnetic field is generated in the vicinity of the main magnetic pole by externally passing a high-frequency current in a microwave band through the current wire, the generated magnetic field being superimposed on a perpendicular recording magnetic field generated from the main magnetic pole to assist magnetization reversal (see U.S. Patent Application Publication No. 2010/0309577A1, for example). This assisted method is referred to as the externally oscillated microwave assisted magnetic recording method.
With the externally oscillated microwave assisted magnetic recording method, it is possible to apply a high-frequency in-plane alternating magnetic field in a microwave band to a recording layer in a magnetic recording medium by superimposing the alternating magnetic field. Due to the resulting assist effect, a perpendicular recording magnetic field from the main magnetic pole can be greatly reduced, the perpendicular recording magnetic field being required for magnetization reversal in the recording layer. Even if the recording layer has a large coercive force Hc, it is also possible to record data in and delete it from the recording layer at high speed. In addition, the alternating magnetic field in the microwave band in the in-plane direction is generated by externally passing a high-frequency current through a current line provided in the vicinity of the main magnetic pole (externally oscillated microwave assisted magnetic recording), so features not found in self-oscillating microwave assisted magnetic recording by the STO are obtained. For example, these features are: the frequency of high-frequency current can be highly precisely controlled; a magnetic field generated in an plane can be easily controlled by controlling the amplitude of high-frequency current; due to these features, a frequency in a microwave band and the strength of a generated in-plane alternating magnetic field can be controlled independently, so it is possible to set a frequency adequate to the strong-magnetization resonant frequency of a magnetic particle in the recording layer and optimally design a magnetic head in consideration of optimum apportionment between the in-plane alternating magnetic field and the vertical recording magnetic field; and the structure of the magnetic head is simple, so its mass production is easy.
In U.S. Patent Application Publication No. 2010/0309577A1, a magnetic head is proposed that is structured so that a wire 111, which generates a microwave magnetic field, is embedded under the end surface of the pole tip103 and the microwave magnetic field is superimposed on the data-encoded magnetic field 201 generated from the pole tip103 (see FIGS. 1 to 3 in U.S. Patent Application Publication No. 2010/0309577A1). In this magnetic head, the data-encoded magnetic field 201 generated from the pole tip103 and the microwave field 211 generated from the wire 111 are superimposed in the targeted bit area 220, enabling microwave assisted magnetic recording.